In many cases, adhesion of a substarte to an adherend comprised of a material different from the substrate is difficult.
For example, a shaped article made of a hydrocarbon polymer, especially a polyolefin resin such as polypropylene or polyethylene, is used in combination with other different materials such as metals or fabrics for automobile parts, household appliances and construction materials. Further, a polyolefin resin used as a corrosion resistant material and a light-weight structural material is used as a laminate with a steel sheet or an aluminum sheet. However, polyolefin resins such as polypropylene and polyethylene are non-polar and have a low solubility parameter (small sp value), and hence, are difficult to adhere to different materials such as metals, for example, iron and aluminum, and glass, ceramics, fabrics and polar polymers.
To enhance the adhesion of a non-polar hydrocarbon polymer, a proposal has been made wherein one or both of the non-polar hydrocarbon polymer and a different material to be adhered, are pre-treated, or coated with a primer. However, a high adhesion strength was difficult to obtain by this proposed method, and, the adhesion effect varies depending upon the material compositions of the particular combination of materials to be adhered, and the applicable adherend materials are limited.
As another attempt for adhesively bonding a polyolefin resin to a different material without pre-treatment or coating with a primer, a composite article has been proposed which is made by adhering a polyolefin shaped article to a different material having a polar group via an adhesive layer comprised of chlorinated polypropylene having a chlorine content of 20 to 30% by weight, a styrene-ethylene-butylene-styrene block copolymer and a hydrogenated terpene resin (Japanese Examined Patent Publication [hereinafter abbreviated to as “JP-B”] No. H6-94207). However, in the case when the polyolefin shaped article to be adhered is made of a crystalline polyolefin resin such as polyethylene or polypropylene, the adhesion strength is still not high. Further, in the case when the different material having a polar group to be adhered to the polyolefin shaped article has a relatively small sp value such as polyethylene terephthalate or polymethyl methacrylate, the adhesion strength is also not satisfactory.
Polyolefin rubbers such as an ethylene-non-conjugated diene copolymer (EPDM), and polyolefin resins such as polyethylene and polypropylene are non-polar and have a low solubility parameter (small sp value), and hence, these polyolefin rubbers and resins are difficult to adhere to a polar polymer having a large sp value. Especially crystalline hydrocarbon polymers such as polypropylene and polyethylene have a high degree of crystallinity, and hence, are difficult to adhere to polyolefin elastomers and non-crystalline polyolefin resins even though these polyolefin elastomers and non-crystalline polyolefin resins have a sp value approximately equal to those of the crystalline hydrocarbon polymers.
To enhance the adhesion between polyolefin rubber and resin compositions, several proposals have been made, which include, for example, a method wherein an adhesive surface of vulcanized EPDM rubber is subjected to a blasting treatment to be thereby roughened, and an adhesive and a thermoplastic resin powder are applied to the roughened surface, and then, a polyester elastomer or a polyolefin elastomer is adhered onto the adhesive-applied surface (Japanese Unexamined Patent Publication [hereinafter abbreviated to as “JP-A”] No. H6-47816); a method wherein a carboxyl group-containing compound or talc is incorporated in a resin composition to be adhered to a polyolefin rubber (JP-A H9-171351); a method wherein an ethylene-1-octene copolymer is incorporated in a thermoplastic elastomer, and the resulting mixture is adhered to a polyolefin rubber (JP-A H9-40814); a method wherein a resin having a polar group such as a hydroxyl group is used as the resin composition to be adhered to a polyolefin elastomer (JP-A H8-244068). However, even though these proposals are adopted, the adhesion strength is not sufficiently high and the kind of resin compositions to be adhered to a polyolefin rubber is limited.
Further, a method has been proposed wherein a polar resin such as a polyamide resin or an ABS resin is adhesively bonded to a vulcanized rubber such as isoprene rubber or ethylene-propylene rubber with an adhesive comprising as the basic ingredient a mixture of polyvinyl alcohol modified with an aldehyde, or a resol-type phenolic resin. This proposed method has a problem such that the adhesion strength is still not sufficiently high and the phenolic compound is not satisfactory from a viewpoint of environmental health.
It is known that isoprene rubbers such as natural rubber and synthetic isoprene rubber are easily cyclized in the presence of an acid catalyst. Several attempts have been made for applying the cyclized isoprene rubbers for a coating material.
For example, a printing ink or coating composition applied for a polyolefin film or shaped article has been proposed which comprises as a vehicle resin a cyclized rubber made by cyclizing a conjugated diene polymer or copolymer having a low molecular weight, and optional ingredients such as a plasticizer and a dispersing agent (JP-A S51-12827).
Further it is taught in JP-A S51-12827 that, when a surface of a polyolefin shaped article is treated with a composition comprising an ingredient selected from a halogenated polyolefin, a halogenated rubber and a cyclized rubber, and a mixed solvent containing a terpene solvent, good coatability and printability and good adhesion to an adherend can be imparted to the surface thereof.
Another proposal of making a cyclized rubber by cyclizing a low-molecular-weight diene polymer has been made wherein a modified isoprene rubber prepared by modifying a low-molecular-weight isoprene polymer having a high cis-1,4-bond content with maleic anhydride or its derivatives is used as the rubber to be cyclized (JP-A S57-145103). It is described in this patent publication that the resulting cyclized rubber has improved adhesion to a polyolefin resin and improved compatibility with a polar resin, and therefore, is suitable for a printing ink and a coating liquid.
As explained above, a conjugated diene polymer having cyclic structures and modified polymers thereof have been proposed for use as a material for a coating liquid or an ink to be applied for a polyolefin resin. However, the adhesion has not been improved to the desired extent, and hence, a further improvement is eagerly desired.